Margaret Kingston Tivey, Associate Scientist
Marine Chemistry & Geochemistry Department
Woods Hole Oceanographic Institution In late summer of 1984, I anxiously awaited my first trip to the
seafloor in the submersible Alvin. There was a
delay in launching the sub, but I resisted the urge to have a drink,
anticipating one final trip to the bathroom before crawling into Alvin’s
three-person, 6-foot sphere for eight hours. I was excited not only
about my first chance to dive, but about visiting the home of the
seafloor rocks I had long been studying for my master’s thesis.
Since 1982, I had spent most of my waking hours examining pieces of
seafloor vent deposits that had been recovered during a routine
dredging operation along the Juan de Fuca Ridge off the Pacific
Northwest coast. Expecting to find common seafloor rocks called
basalts, scientists were surprised to pull up fragments and boulders of
massive sulfide covered with small tubeworms. They had discovered the
fourth and, at the time, newest site of hydrothermal venting on the
seafloor, a place now known as the Main Endeavour Field. These rocks
helped launch my scientific career.
In the years leading up to
my 1984 dive, I had learned that hydrothermal vent systems played a
significant role in transferring heat and mass from the solid Earth to
the ocean, and that the vent sites host unusual biological communities,
including tubeworms, bivalves, crabs, and fish that thrive in the
absence of sunlight. It was also becoming clear that the relatively
constant chemistry of the ocean was in part sustained by hydrothermal
activity.
What my colleagues and I were only beginning to
realize then was that hydrothermal vent systems are like snowflakes—no
two are ever exactly alike.
Long-standing mysteries
When scientists in Alvin
discovered the first active hydrothermal system in 1977 at the
Galápagos Rift, they found warm fluids, later determined to be a blend
of cold seawater and hot vent fluids, seeping from seafloor crust.
Never-before-seen organisms were present at the vents, including large
clams and tall, red-tipped tubeworms.
In 1979, a second
active hydrothermal system was discovered along the East Pacific Rise.
At that site, much hotter fluids (350°C) jetted from tall rock
formations composed of calcium sulfate (anhydrite) and metal sulfides.
When the clear hot fluid jetting from these chimney-like structures
mixed with cold seawater, fine particles of dark metal sulfides
precipitated out of solution, creating the appearance of black “smoke”
(hence the name “black smoker chimneys.”)
The discovery of
these vent systems immediately answered a question long posed by
geophysicists: How is heat transferred from Earth’s interior to the
oceans? Earlier studies had shown that, contrary to model predictions,
not as much heat was being transferred by conduction
(particle-to-particle transfer) near the ridge crests.
Scientists hypothesized that heat was also transferred by convection,
as fluid circulated within the crust near mid-ocean ridges. Sure
enough, cold seawater is entering cracks and conduits within seafloor
crust. It is being heated by underlying rocks and rising and venting at
the seafloor, carrying significant amounts of heat from Earth to ocean.
The chemistry of the ocean
The discovery of vents allowed scientists to begin to answer another
major question: How does the ocean maintain its relatively constant
chemical composition? Over time, rivers drain materials into the
oceans, and winds blow in particles, some of which dissolve to add
chemical elements to the oceans. Some of these elements, in turn, exit
ocean waters, settling in ocean sediments, for example.
Many
components of seawater—including lithium, potassium, rubidium, cesium,
manganese, iron, zinc, and copper—enter the oceans via vents. They are
leached from seafloor crust by subterranean chemical reactions with hot
hydrothermal fluids. Other hydrothermal vent reactions draw elements
out of seawater and place them back into the earth.
For
example, magnesium eroded from land is carried to the ocean by rivers.
Yet magnesium concentrations have not increased in the oceans.
Scientists puzzled for decades over where all the magnesium could be
going.
Scrutinizing hydrothermal vents, researchers found
that seawater entering seafloor crust is rich in magnesium, but fluids
exiting the vent are free of it. Oceanographers surmised that magnesium
is left behind in the crust, deposited in clay minerals as seawater
reacts chemically with hot rock.
Dive to Main Endeavour Field
In those early years when observations were few and samples fewer, my
thesis rocks provoked considerable interest. In some ways, the rocks
were similar to those recovered from the East Pacific Rise, but in
other ways they were quite different.
The Main Endeavour
Field samples were rich with amorphous silica, which should only
precipitate if the hydrothermal fluid had cooled without mixing with
seawater. And they did not contain anhydrite, a common vent chimney
mineral that dissolves in seawater at temperatures less than about
150°C (302°F).
We theorized that the dredged pieces must
have come from the low-lying mounds that lay beneath and around black
smoker chimneys. Our dives to the Endeavour Field in 1984 would tell us
if we were correct.
The trip to the seafloor took 90
minutes. As we approached the seafloor, the pilot asked me to look out
my viewport and let him know when I saw bottom. Alvin’s
lights were turned on. It was like the curtain going up in a dark
theater and the stage lights going on. We were hovering over the same
basaltic rocks that I had spent countless hours studying in photographs.
Then to my right, I could see a rock wall rising from the seafloor. It
was obvious from the hedges of pencil-diameter tubeworms sticking out
of the tops and sides of the cliffs that I was looking at large
hydrothermal vent structures.The view was nothing like what had been
described at the East Pacific Rise. Instead of low-lying mounds of
sulfide debris topped by active smokers, we saw steep-sided structures
standing 15 meters (50 feet) high.
Why was this site so very different? How could these chimneys stand there like multi-story buildings without collapsing?
A fluid environment
Answers to these questions came from studying new samples. We learned
that the tall chimneys structures were essentially “cemented”—silica
filled pores in the vent structure walls as the emerging fluids cooled,
making the structures sturdy. But why was so much silica precipitating
at this site?
The fluid chemistry provided answers: This
vent site’s fluids were rich in ammonia (NH3). As the ammonia-rich
fluids cool, NH3 takes up excess H+ ions to form NH4, which raises the
pH of the fluids. The higher pH likely allows silica to precipitate
within Main Endeavour Field structures; at sites with no NH3, low pH
probably inhibits the formation of amorphous silica.
As with
almost every visit to a new vent site, our survey of the Main Endeavour
Field raised as many questions as it answered. The differences among
known hydrothermal systems and the revelations that accompanied each
new discovery provoked oceanographers to hunt for new sites. We were
explorers trying to learn as much as we could. Hypotheses were
advanced, only to be proven wrong by yet another discovery.
More visits to these seafloor hot springs made it clear that all vent
fluids are not the same. Rather, the chemical composition changes from
ridge to ridge—and from time to time.
Researchers returning
to some vents found that the chemistry of vent fluids was not constant,
changing on scales ranging from days to years. These vent sites were
all associated with sites of recent magmatic activity, with recorded
earthquakes and evidence that dikes had been intruded into the ocean
crust and, at some sites, that lava had been extruded onto the
seafloor. The vent fluid compositions were changing as these dikes
cooled, and as fluids penetrated deeper into the crust.
Searching for new vents
In the early 1980s, after a number of vent sites had been found in the
Pacific, scientists began to wonder if hydrothermal activity and active
black smokers might exist on the more slowly spreading Mid-Atlantic
Ridge. The hydrothermal vent systems transfer large amounts of heat
from magma or newly solidified hot rock, but on slow-spreading ridges
the spreading rate, and magma delivery rate, is much less (about 1/3)
of that on the northern East Pacific Rise and Juan de Fuca Ridge.
To our surprise, exploration from the mid-1980s through the early 1990s
gradually made it clear that hydrothermal systems may be spaced further
apart on the Mid-Atlantic Ridge, but they tend to generate much larger
mineral deposits. Expeditions to the Southwest Indian Ridge in 2000 and
the Gakkel Ridge (under the Arctic Ocean) in 2001 revealed that
hydrothermal venting occurs on even the slowest-spreading portions of
the mid-ocean ridge system.
New ways to find vents
Two decades of study have taught us that there is no single type of
seafloor hydrothermal vent system. The plumbing systems beneath the
seafloor are both diverse and incredibly complex.
Ocean
scientists today are posing questions about the dimensions and
evolution of the hydrologic systems beneath vent sites. We puzzle over
how hot these fluids get, how deep into the crust they descend, and how
far they travel before venting at the seafloor. And where does seawater
enter these systems?
To answer these questions, we will need
to continue exploring, not only over geographic space, but also over
time. In the early years, most vents were discovered serendipitously,
but as we’ve explored and learned more about these systems, we’ve been
able to develop systematic methods for pinpointing sites.
For example, a technique of “tow-yowing” has been developed, where a
conductivity-temperature-depth (CTD) sensor is raised and lowered
through the water column in a saw-tooth pattern above the ridge to map
the locations of plumes, and then to home in and map the buoyant
portion of plumes coming directly from active vent sites. This
technique was used successfully to find vent sites in the Pacific and
Atlantic, and most recently in the Indian Ocean.
Seafloor observatories
The need to explore the dynamics of hydrothermal systems over time has
led to new technologies and the development of seafloor observatories.
New, more precise and durable instruments allow us to monitor
temperature and fluid chemistry at vent sites for hours, days, or
months—as opposed to observing those properties for brief moments and
grabbing one-time samples.
The future of hydrothermal studies
was displayed in a recent series of coordinated experiments. With
support from the National Science Foundation’s Ridge Interdisciplinary
Global Experiments (RIDGE) program, a team of researchers built a
seafloor observatory on the Endeavour segment of the Juan de Fuca
Ridge.
During the summers of 2000 and 2001, scientists made
complementary and continuous observations centered around the Main
Endeavour Field (the same site I first visited in 1984) and at vent
sites to the north and south. The program goals included making more
accurate measurements of the heat and mass flowing from the system, and
observing how the hydrothermal plumbing is influenced by tides and by
high-temperature reactions that separate elements into saltier liquids
and more vapor-rich fluids (a process called “phase separation”).
Instruments were deployed to continuously monitor vent fluid
temperatures, flow rates, and chemical properties. Scientists also used
newly developed samplers to collect fluids at regular time intervals.
While these instruments were in place, other researchers made acoustic
images of vent structures and venting fluids. Still others used the
Autonomous Benthic Explorer (ABE) to measure water column properties
above the vent field, seafloor depth, and magnetic signatures.
Later in the program, the team deployed a systematic array of current
meters, thermistor strings, magnetometers, and tilt meters. Scientists
even tested techniques to “eavesdrop” on the data being collected and
download it without removing the instrument from the vent. The result
of this collective effort was the most comprehensive study of a
hydrothermal system to date, and a model for future seafloor
observatories.
A continually unfolding story
As we develop these new techniques and instruments, our ability to
explore ongoing seafloor processes will grow. More than a
quarter-century into our studies, we still find ourselves constantly
revising and refining our ideas about hydrothermal systems.
At the same time, as we home in on the fine details of how these
systems work, we continue to find new sites that completely break the
mold. As recently as December 2000, researchers diving in the
Mid-Atlantic discovered “Lost City,” a vent site located far away from
the ridge axis, on old rather than nascent seafloor crust, and with
15-story-high white minaret-like structures made of carbonate—a mineral
that is not found at most other known vent sites.
So after 32
dives to the seafloor to study vents, I am often still surprised, and I
am always awed. Even when I return to a vent site that I’ve visited
before, I still find it an unbelievably beautiful sight to watch jets
of hot fluid mixing with seawater, and unusual organisms that make
their homes near these vents. Like my colleagues, I look for ways to
make our studies more precise, more methodical, and more continuous.
But 20 years after my first dive, I still enjoy seeing it all live.
It’s the difference between watching a movie of a waterfall and
standing next to one.
Posted: February 13, 2004 [top] |
